1. Field of the Invention:
This invention relates generally to dew point hygrometers and more particularly to dew point hygrometers using polarized light and a condensation surface with a temperature gradient in the plane of the surface.
2. Description of the Prior Art
It is well known that the dew point of a moisture containing gas is a function of the amount of moisture contained in the gas. The dew point hygrometer makes use of this concept to determine the water content of a gas. Gas is passed over a surface as the surface is cooled until condensation forms on the surface. The temperature of the surface is then measured to determine the dew point temperature of the gas which can be correlated to the moisture content of the gas.
This method has been the most accurate technique for determining moisture content of a gas and is well known. However, prior art devices have determined the presence of condensation by reflecting light off the surface and measuring the relative amounts of diffuse and specular reflection. Without condensation, the polished metal surface produces only a specular reflection. Condensation normally produces a diffuse reflection.
However, under certain circumstances, such as at temperatures below -60.degree. F., the condensation forms as glaze ice on the surface. Glaze ice gives a specular reflection similar to the metal surface, rather than a diffuse reflection. As a result, inaccurate results are produced by this method when glaze ice is formed.
Also, even in the absence of glaze ice, certain inaccuracies in measuring the temperature are encountered. The metal surface is typically cooled from below, resulting in a temperature gradient through the thickness of the metal, normal to the surface. Since the temperature of the metal is measured at a point below the surface, the measured temperature will be slightly different from the temperature at the surface. Further, since it is not possible to detect when condensation is imminent, but only possible to detect it after it has happened, the forming condensation adds a further temperature gradient. Finally, there is a gradient across the thickness dimension of the gas in laminar flow and within the boundary in turbulent flow. The result of this is that the measured temperature is not exactly the temperature of the gas.